Wi-Fi Assist is still present in iOS 9.3 beta 1. The difference is a tiny counter underneath the toggle to enable Wi-Fi Assist, which tells you how much data has been used by the feature.

It’s not a perfect fix. For instance, I would still like some kind of symbol in the status bar (or a drop-down notification) when mobile data is being used in preference of Wi-Fi. But at least with this counter, I’ll be able to see if iOS is really the culprit for me running over my data allowance, rather than just guessing.